initramfs: source user scripts from /e/z/initramfs-tools-load-key{,.d/*}

By dropping in a file in a directory (for packages) or by making a file
(for local administrators), custom key loading methods may be provided
for the rootfs and necessities.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Nicholas Morris <security@niwamo.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Co-authored-by: Nicholas Morris <security@niwamo.com>
Supersedes: #14704
Closes: #13757
Closes #14733